200 



CHAPTER III. 



128 



MUSCULAR SYSTEM OF H2EMATOCRYA. 



45. Structure of Muscle. Muscular tissue is fibrous, and resol- 

 vable into fine threads inclosed in a delicate sheath, called 4 elemen- 

 tary fibres.' These, in Vertebrates, are of two kinds ; in one the 

 fibre is crossed by close parallel lines ; in the other it is smooth. 

 The transversely striped character is too fine to be seen without the 

 aid of the microscope ; but may be indicated to the naked eye by 

 the iridescence of the surface in certain lights. 1 All the muscles 

 subject to the influence of the will, or cerebral action, have striped 

 fibres. Most of the involuntary muscles have unstriped fibres ; 

 those of the heart and gullet are among the exceptions ; and, on 

 the other hand, the muscles performing the rhythmical movements 

 of the gill-covers in fishes, like those of the thoracic walls in 



higher air-breathers, have the 

 striped fibre. But besides the close 

 cross parallel lines, longitudinal 

 ones, darker, wider apart, and of 

 varying extent, often present them- 

 selves on the elementary fibre of 

 voluntary muscle, as in fig. 128, 

 A a.~ 



The fibre, though termed l ele- 

 mentary ' may, by manipulation 

 and chemical agency, be resolved 

 into parts of different forms. 3 It 



Portions of striped elementary fibres, showing 



a cleavage in opposite directions, magnified 8661118 lllOSt prone to Split llltO lon- 

 300 diam. CLXXXV. , . , i i i i 



gitudmal tracts, which nave been 



termed ' fibrils,' fig. 128, A, b and c, and these have a show 

 of segments equalling in length the breadth of the transverse 

 striae. Sometimes such segments appear by alternate dark and 

 light parts of a continuous rectilinear fibril, as in the upper por- 

 tion at c, fig. 128. Sometimes the segments are marked off by 



1 xx. vol. i. p. 10. 2 CLXXXV. p. 508. 



Ib. 



